Gradstein, F. M., Ludden, J. N., et al., 1992 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 123 36. ARGO ABYSSAL PLAIN MAGNETIC LINEATIONS REVISITED: IMPLICATIONS FOR THE ONSET OF SEAFLOOR SPREADING AND TECTONIC EVOLUTION OF THE EASTERN INDIAN OCEAN1 William W. Sager,2 Lawrence G. Fullerton,3 Richard T. Buffler,4 and David W. Handschumacher5 ABSTRACT Linear magnetic anomalies in the Argo Abyssal Plain have been interpreted as having been recorded by seafloor spreading during Late Jurassic to Early Cretaceous Chrons M26 through M16. Ocean Drilling Program Leg 123 drilled at Site 765 in the southern Argo Abyssal Plain, near the base of the northwest Australia margin between anomalies thought to be M25A and M26. However, initial biostratigraphy of sediments overlying basement gave Early Cretaceous ages, ~20 m.y. younger than expected. With this discrepancy as impetus, we re-examined the magnetic lineations in the Argo Abyssal Plain and decided that the best model is still the sequence M26 through Ml6. In addition, we were also able to construct a model that accounts for most of the lineations with the reversal sequence MO through Mil, closer to the basement age predicted by initial biostratigraphic results from the deepest sediments at Site 765. This model proved unsatisfactory because it left a significant portion of the lineations unexplained, requires an unlikely sequence of tectonic events, and disagrees with a reliable Jurassic radiometric age that has been determined from Site 765 basement basalts. Later biostratigraphic studies caused the ages of the oldest sediments at Site 765 to be revised upward, but not enough to eliminate the discrepancy with the basement age inferred from the magnetic lineations. A 5-10 m.y. difference exists between oldest sediments and basement at Site 765, whereas the discrepancy at nearby Site 261 is 3-8 m.y. The probable explanation is that sedimentation on the Jurassic Argo Abyssal Plain was low because the northeast Australian margin was sediment-starved and rugged, allowing little sediment to reach the Argo basin. However, some of the discrepancy may arise from small inaccuracies in the Jurassic geomagnetic polarity reversal time scale or small ridge jumps in the young Argo Abyssal Plain. Our Argo magnetic lineation map implies a relatively simple tectonic history for the basin. Seafloor spreading began shortly before M26 time along the center of the northwest Australian margin and extended east and west through ridge propagation. An initially-segmented Argo spreading center coalesced into fewer, longer spreading segments until ~M21-M19 time when a global plate reorganization caused the ridge to resegment. Spreading began on the western margin of Australia at M10 time in the Early Cretaceous, but does not appear to have been contemporaneous with the observed period of spreading in the Argo basin. INTRODUCTION Before ODP Leg 123, the available drilling results from the abyssal plains off northwest Australia seemed in agreement with The Argo Abyssal Plain, a salient of Indian Ocean lithosphere the ages of the anomalies found there (Table 1; Fig. 3). Leg 123 wedged between Australia and the Java Trench, is one of several drilled at two sites, 765 and 766, on the southern edge of the Argo deep ocean basins that border northwest Australia (Fig. 1). Meso- Abyssal Plain and southwest Exmouth Plateau, respectively (Fig. zoic magnetic lineations are found in these basins, indicating that 2). Though the oldest sediments recovered at Site 766 were in they were formed by seafloor spreading. In the Gascoyne, Cuvier, accord with the age of the seafloor predicted by the magnetic and Perth abyssal plains, located to the west of the Exmouth anomalies, discordant results were obtained at Site 765 in the Plateau and western Australia, these lineations generally have a Argo Abyssal Plain, drilled between anomalies M25 and M26 trend of N30°E, but in the Argo Abyssal Plain, the trend is N70°E near the base of the continental rise (Fig. 2). Over 935 m of (Fig. 2). Moreover, the anomalies also indicate a difference in age. sediments and 270 m of igneous rocks were recovered at the site, The Early Cretaceous sequence M0-M10 (118-132 Ma) is found and the shipboard scientific party concluded that basement had in the Gascoyne, Cuvier, and Perth basins, but Late Jurassic been penetrated. Initial biostratigraphic studies of sediments re- anomalies M16-M26 (144-163 Ma) have been identified in the covered immediately above basement indicated late Berriasian to Argo Basin (e.g., Fullerton et al., 1989; ages from Harland et al., Valanginian stage, Early Cretaceous ages rather than an Oxfor- 1982). Thus, the Argo Abyssal Plain is one of the few remaining dian stage, Late Jurassic age as expected from the magnetic parcels of Jurassic-age seafloor in the oceans. Indeed, it is this lineations (Ludden, Gradstein, et al., 1990). antiquity that makes it of special interest to oceanographers and This discrepancy seemed difficult to reconcile as it implied partially prompted drilling at Ocean Drilling Program (ODP) Site that no sediments accumulated in the Argo Abyssal Plain for -20 765. m.y. after the beginning of seafloor spreading. Indeed, the ship- board scientific party of Leg 123 found this scenario unappealing and suggested that a reappraisal of the magnetic lineations and their tectonic implications was warranted (Ludden, Gradstein, et al., 1990). They proposed that previous assessments of the age of Gradstein, F. M., Ludden, J. N., et al., 1992. Proc. ODP, Sci. Results, 123: the central Argo Abyssal Plain lithosphere might be erroneous, College Station, TX (Ocean Drilling Program). Departments of Oceanography, Geophysics, and Geodynamics Research Insti- perhaps as a result of incorrect interpretation of the lineation tute, Texas A&M University, College Station, TX 77843, U.S.A. identities and trends. Indeed, the new inferred basement age for 3 Landon School, 6106 Wilson Lane, Bethesda, MD 20817, U.S.A. Site 765 implied that the Argo Abyssal Plain might have formed 4 University of Texas Institute for Geophysics, 8701 North Mopac Blvd., Austin, at the same time as the Gascoyne, Cuvier, and Perth abyssal plains TX 78759, U.S.A. Naval Oceanographic and Atmospheric Research Laboratory, Stennis Space rather than being much older as proposed by previous geophysical Center, MS 39529, U.S.A. investigations (e.g., Fullerton et al., 1989). This situation 659 W. W. SAGER, L. G. FULLERTON, R. T. BUFFLER, D. W. HANDSCHUMACHER PERTH ABYSSAL PLAIN Figure 1. Generalized bathymetric map showing the study area and geographic features mentioned in text. 1, 3, and 5 km contours are shown (based on GEBCO charts 5-09 and 5-10). prompted us to take a fresh look at the magnetic anomalies and to datum implies that the Jurassic magnetic lineation model for the test the hypothesis that these anomalies correspond to magnetic Argo Abyssal Plain anomalies is probably correct. Rather than reversals of the Early Cretaceous. delete the Early Cretaceous lineation model entirely, it is left as Though we were able to make a match of Early Cretaceous an illustration of the process and pitfalls of reconciling the ages anomalies M0-M11 with some of the magnetic lineations, our of magnetic lineations with overlying sediments. study convinced us that the previous Jurassic model is best. While this article was in review, biostratigraphic ages for the oldest Geologic Setting sediments at Sites 765 and 261 were revised (Table 1; Fig. 3). At The Argo Abyssal Plain is bounded on two sides, south and Site 765, the age was revised upward, from late Berriasian-Val- east, by the continental crust of Australia and to the north, by the anginian to Tithonian (Dumoulin and Brown, this volume; Mut- Java Trench. The continental margin is an Atlantic-type rifted terlose, this volume), decreasing the discrepancy between the margin draped with only a thin blanket of sediments (Falvey and biostratigraphic and geomagnetic polarity time scale age esti- Veevers, 1974; Powell, 1976). Extension, uplift, and erosion mates from about 20 to 10 m.y. However, the biostratigraphic ages occurred along this part of Australia during the Triassic, but for sediments overlying basement at Site 261, first thought to be breakup, subsidence, and crustal thinning began in earnest during late Oxfordian age (Veevers, Heirtzler, et al., 1974), were revised the Middle Jurassic (Powell, 1976; Veevers and Cotterill, 1978). downward to Kimmeridgian-early Tithonian (Dumoulin and It is generally accepted that this rifting event carved a continental Brown, this volume; Mutterlose, this volume), creating a 3-8 m.y. block from northwest Australia, although the present identity of discrepancy between the basement ages estimated by biostra- this block is not certain (Larson, 1975). To the southwest of the tigraphy and the magnetic polarity time scale (Fig. 3). Even more Argo Abyssal Plain, the margin includes the stretched continental recently, an Ar^-Ar39 radiometric age of 155±3 Ma was deter- crust of the Exmouth Plateau and to the east, the Scott Plateau mined by R. Duncan using celadonite from Site 765 basement (Falvey and Veevers, 1974). Because sediments on the continen- basalts (F. Gradstein and J. Ludden, pers. comm., 1991). This tal slope and rise are thin, the Continent-Ocean Boundary (COB) 660 ARGO ABYSSAL PLAIN MAGNETIC LINEATIONS IO5°E Figure 2. Magnetic lineations adjacent to the northwest Australian margin. (Stippled regions) bathymetry <5 km. (Heavy lines) fracture zones;